Coherent spectroscopy for TSVs

Abstract

There is provided a system and method for OCD metrology, comprising: providing an interferometric mirror of the system at each of a plurality of positions z, where at each position a mirror reflection matches a reflected light path difference (OPD) from at least one reflective surface, and measuring an interferometer spectrum Imeasure with the mirror at each of the plurality of positions; fitting the spectra of the plurality of measurement interferometers to an Imeasure equation to solve non-z correlation parameters of the equation, and leaving a z correlation function with a complete coherence term and a partial coherence term of a wave number k; and removing the partial coherence terms of the function to derive a full coherence field for characterizing the OCD structure.

Description

Technical Field

[0001] The present invention relates generally to the field of optical inspection of integrated circuit wafer patterns, and in particular to algorithms for measuring wafer pattern parameters. Background Art

[0002] Integrated circuits (ICs) are produced on semiconductor wafers through multiple steps of depositing, altering, and removing thin layers, which are accumulated on the wafer into stacked structures. These stacked structures (also called "stacks" or "features") can be formed in a pattern that has optical properties similar to a diffraction grating.

[0003] Optical critical dimension (OCD) metrology uses scatterometry, which measures the optical radiation reflected from structures formed on a sample (i.e., wafer) during IC production. Common scatterometry methods include spectroscopic reflectometry (SR), spectroscopic ellipsometry (SE), and spectroscopic interferometry (SI). Scatterometry is often applied to OCD metrology during IC production to determine whether the wafer pattern was made with the correct (i.e., valid) parameters. The measurement can determine the degree of variation from the design specifications. The manufacturing protocol can specify the allowed deviation from the mean value.

[0004] Scatterometry methods face serious challenges when measuring thick structures, especially those with several reflective surfaces that are clearly separated from each other. A common application with these characteristics is the fabrication of “through silicon vias” (TSVs), interconnect structures with dimensions reaching tens of micrometers or even exceeding 100μm. Other examples are ultra-thick dielectrics, CMOS image sensors, and various structures used in advanced semiconductor packaging.

[0005] The interference of reflections from surfaces at different heights results in oscillations of the measured reflection as a function of wavelength, i.e. the reflection changes significantly for very small wavelength differences. When the differences are large, the measured spectrum may be "smeared" by spectral features that cannot be resolved by the measurement. Such a situation results in a loss of sensitivity and applicability of the metrology solution.

[0006] In order to resolve the spectral features for such structures, the measurement equipment (e.g., spectrometer) may need to have extremely high spectral resolution. However, this increases cost and complexity, and still cannot solve the problem of very thick structures. In addition, such solutions are not scalable, and as semiconductor applications become thicker, high-end spectrometers cannot keep up with the required spectral resolution.

[0007] In such a situation, the measured spectrum is "smeared" with spectral features that cannot be resolved by the measurement. Such a situation results in a loss of sensitivity and applicability of the metrology solution.

[0008] One possible approach involves using longer wavelengths, namely infrared and mid-infrared. This results in slower oscillations at longer wavelengths, but also in a loss of sensitivity, since higher wavelengths distinguish certain structures better. Longer wavelengths also increase system complexity, increase measurement time (due to lower brightness sources and less efficient detectors), and increase the necessary measurement spot size (due to diffraction at longer wavelengths).

[0009] Measuring with a small numerical aperture can improve results, since the angular span of the incident light is another factor that causes coherence loss. By reducing this angular span, spectral contrast can be increased. Of course, such a limitation has a direct negative impact on light throughput, resulting in increased noise.

[0010] Another approach to the spectral resolution challenge is provided by monochromator-based solutions, where a scanning element measures scattered light of a specific wavelength at any given moment. High spectral resolution can be achieved, but at the expense of long measurement times, which are generally unsuitable for process control and high-throughput metrology.

[0011] Another alternative for increasing spectral resolution is provided by Fourier-based methods. In such methods, the integral over a wide spectral range is measured, but using a scanning element (usually a mirror), different measurement instances capture differently weighted sums of the signal. Methods in this category are Fourier transform IR (FTIR) and white light interferometry (WLI). In general, the final spectral resolution is proportional to the range swept by the scanning element, allowing very high spectral resolution. However, as mentioned before, high resolution comes at a direct cost of measurement time. Summary of the invention

[0012] According to embodiments of the present invention, embodiments of the present invention provide systems and methods for optical critical dimension (OCD) metrology based on optical path difference (OPD) matching interferometry. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] In order to better understand the various embodiments of the present invention and to show how to implement the various embodiments of the present invention, reference is made to the accompanying drawings by way of example. The structural details of the present invention are shown to provide a basic understanding of the present invention, and the description combined with the accompanying drawings makes it obvious to those skilled in the art how to embody several forms of the present invention in practice. In the accompanying drawings:

[0014] Figure 1 is a schematic diagram of a system for optical critical dimension (OCD) metrology using optical path difference (OPD) matching interferometry according to an embodiment of the present invention;

[0015] Figure 2A and Figure 2Bis a diagram showing the effect of partial coherence on signal amplitude;

[0016] Figure 3 is a schematic diagram showing spectroscopic examination of a composite body having multiple surfaces including one surface located at a depth beyond the range of full coherence with higher surfaces;

[0017] Figure 4 is a plot of the Fourier transform of the field in the wavenumber domain (or frequency domain) to the time domain impulse response; and

[0018] Figure 5 is a flow chart depicting a process for characterizing OCD metrology using OPD matching interferometry in accordance with an embodiment of the present invention. DETAILED DESCRIPTION

[0019] Embodiments of the present invention provide systems and methods for optical critical dimension (OCD) metrology based on optical path difference (OPD) matching interferometry when the distance between surfaces results in partial coherence.

[0020] Figure 1 is a schematic diagram of a system 20 for spectroscopic interferometry metrology of a wafer (shown as sample 22) wherein there are at least two reflective surfaces to be measured, namely a top surface 24 and a bottom surface 26 separated by a distance H.

[0021] Light from a light source 30 (usually having a predetermined wavelength range) passes through a first beam splitter 32 and is directed to the structure to be measured. The light is then split by a second beam splitter 34, with one portion of the light directed to the sample and the other portion directed to a mirror 36.

[0022] After reflection from the sample and the mirror, the light from both is recombined for collection by spectrometer 40. SI measurement involves collecting several spectra at different positions of the interferometer mirror 36.

[0023] In an embodiment of the invention, the mirror position is "matched" to the reflective surface of the structure being measured so that the optical path difference (OPD) of the reflected light is equal to zero. For example, OPD matching of the mirror to the top surface involves setting the distance B (the distance from the mirror to the beam splitter 34) to the distance A (the distance from the beam splitter to the surface). OPD matching of the bottom surface involves setting B to A+H.

[0024] The optical path length of each path is the product of the geometric length and the refractive index of the material through which the light is propagating. Inside the structure, each electromagnetic eigenmode has a different phase velocity and, therefore, a different effective refractive index and a different OPD. In summary, OPD matching involves tuning the system to a specific value or range of values ​​of OPD.

[0025] The system 20 can be operated within a production line (not shown) for production and monitor the structure of the wafer. Additional details of spectral interferometry (SI) and its basic principles can be found in U.S. Patent No. 10,161,885 and PCT Patent No. PCT / IB2022 / 050774, both in the name of Nova Ltd. and incorporated herein by reference. Additional components (such as polarization control during illumination and acquisition, beam shaping, etc.) can be added to the device, but are ignored here for clarity.

[0026] The additional components of such a system may include an imaging lens, a polarizing filter, a variable aperture diaphragm, and a motor. The operation of such an element is usually automated by a computer controller, which may include I / O devices and may also be configured to perform data processing tasks, such as generating scatterometry data, which may include a spectrogram, also referred to herein as a spectrum. The data points of the spectrum, which may be represented in vector form, are measures of reflected light intensity at different light wavelengths (or wave numbers or frequencies). In a typical OCD measurement, the light range measured may cover the visible spectrum, and may also include wavelengths in the ultraviolet and infrared regions. A typical spectrogram output for OCD measurement may have, for example, 245 data points, covering a wavelength range of 200nm to 970nm.

[0027] When there are reflections from two interfaces (also referred to herein as "surfaces") separated by a thickness H, the reflections acquire a phase difference, which results in an oscillating spectral behavior. Fourier analysis of the reflection spectrum reveals distinct peaks, the spacing between which is proportional to the stack height H.

[0028] The challenges posed by such situations involve the ability of practical measurement systems to resolve these fast spectral features. When spectral oscillations cannot be resolved by the measurement equipment (usually a spectrometer), the resulting data will be smeared, resulting in a loss of spectral contrast. In addition, the angular dependence of the reflected spectrum can be very high; because the measurement involves acquiring a range of incident angles, such high dependence can also lead to smearing of the measured spectrum. Under such conditions, sensitivity is greatly reduced to the point where some properties cannot be measured reliably.

[0029] The effect of spectral tailing (due to any of these effects) can be described in terms of loss of coherence. Due to the limited spectral resolution of the spectrometer, it does not measure the oscillation intensity I(k), but the tail intensity:

[0030]

[0031] where K(k′, k) is a weighting function that describes the details of the spectral tail. For ease of presentation, we take That is, a Gaussian centered at k.

[0032] In the absence of mirror reflectivity, the interference of light reflected from the top and bottom interfaces can be described by the following equation:

[0033]

[0034] However, when H is large enough, the coherent term (cosine) is suppressed. In general, we can use a more general suppression factor γ(k) instead of Gaussian suppression, depending on the exact form of K(k′,k). It can be noted that when γ = 0 (or very close to zero), the resulting intensity involves only the incoherent summation of two reflected terms: I incoherent =|r top | 2 +|r bot | 2

[0035] Spectral tailing results in a loss of coherence, accounting for both the interference between the top and bottom stack reflections (γ) and the reflections from the bottom of the stack and the mirror (γ'), where γ is the coherence of the OPD 2nH and γ' is the coherence of the OPD 2(nH-z). For a Gaussian K(k',k), as well as

[0036] Figure 2A and Figure 2B is a diagram showing the effect of partial coherence on signal amplitude.

[0037] Figure 3 is a schematic diagram of a structure with two reflective regions separated by thickness H. The reflection from the bottom interface acquires an additional phase term, causing the spectral output to be highly oscillatory due to interference.

[0038] Figure 4 Fourier analysis of the reflectance spectrum shown as an impulse response, a time domain function, reveals distinct peaks, with the spacing between the top and bottom surfaces proportional to the stack height H. When the top surface of the structure includes a non-trivial structure with multiple layers, the Fourier transform and There are multiple peaks at z = 0, and multiple frequencies will appear in the reflection spectrum (see Figure 3 ). In such cases, it may be extremely difficult to separate the stack height information from the measurement.

[0039] However, the bottom surface impulse response can be removed from the time domain, thereby removing the signal from the wavenumber domain after further Fourier transformation.

[0040] Figure 5 is a flow chart depicting a computer-implemented process 500 for characterizing a structure to be measured.

[0041] It should be noted that SI measurements provide a complex reflection field Both the amplitude and phase of the reflected field are acquired. The reflected amplitude can be obtained using standard reflection measurements, where the measured intensity I(k) = |r s (k)| 2 Such measurements are very stable and simple to implement.

[0042] However, spectral phase can only be measured using interferometer mirrors and involves taking measurements at a number of mirror positions.

[0043] As mentioned above, it is critical to use the z-related term to extract both the phase and amplitude of the reflected composite field.

[0044] In many important cases, fast (coherent) oscillations are crucial for metrology, as they have direct sensitivity to the parameters of interest. One such case is measuring the depth of a TSV (or the thickness of a thick layer): the standard approach involves using Fourier analysis of the oscillation spectrogram.

[0045] Therefore, the suppression of coherent fields often significantly limits the quality of spectral interpretation.

[0046] The method of the present invention involves using at least two SI measurements when the interferometer mirror is OPD matched to each reflective interface of interest. When the mirror is OPD matched to the top surface, spectral tailing affects the interference at the bottom. However, the interference with the top surface is completely coherent.

[0047] Likewise, when the mirror is OPD-matched to the bottom interface, spectral tailing affects the interference at the top. However, the interference with the bottom interface is completely coherent.

[0048] It is not important whether the mirror is OPD matched at a certain spacing from the top or bottom interface; as long as such spacing is within the "coherence length" of the system. Measurements made with OPD matching at multiple locations within the coherence length will be highly coherent.

[0049] The extraction of the coherence field (ie, the coherence between the mirror and the corresponding surface) may be performed by the following steps of process 500 .

[0050] Step 510, measuring multiple interferometer spectra I measured , where the interferometer mirror is set to different positions z around the point where OPD is zero with respect to the first surface iIn other words, all measurements fall within the full coherence range of the first surface, i.e., within the "coherence length" (so that no "partial coherence" factor is needed). As an example, we can call the highest surface to be analyzed (closest to the light source) the "top" surface. If there are multiple distinguishable height differences between the surfaces, they can all be measured by separate OPD matching.

[0051] Step 520, fitting multiple measured spectra to I measured The mathematical equation can be defined as:

[0052]

[0053] in:

[0054] k is the wave number at each interferometer measurement

[0055] z i is the mirror position for each of the i measurements;

[0056] is the wavelength-dependent reflection phase difference of the corresponding reflection pairs tb (top-bottom), tm (top-mirror) and bm (bottom-mirror);

[0057] H is the geometric difference between interfaces;

[0058] n is the refractive index of the structure;

[0059] r m 、r t 、r b are the reflectivities of the mirror, top surface, and bottom surface, respectively, and are all functions of k;

[0060] γ is the coherence of OPD 2nH;

[0061] γ' is the coherence of OPD 2(nH-z); and

[0062] R is a real function with an imaginary part.

[0063] The coherence γ is defined as the ratio between the measured interference term and the interference expected for a perfectly coherent field (which can be measured, for example, by moving one of the mirrors in a Michelson interferometer and measuring the amplitude of the resulting oscillation).

[0064] The multiple measured spectra are then fitted to the expected functional form by minimizing the following equation:

[0065]

[0066] When the OPD matches the bottom surface (step 530), the fitting equation is:

[0067]

[0068] A similar fitting equation is used for minimization. The non-z-dependent terms can then be cancelled for all mirror positions, leaving the z-dependent functions for the top and bottom, respectively. It can be noted that I measured Some terms of (k,z) can also be derived separately and their values ​​​​substituted into the equation, such as the mirror reflectivity, which is not related to a given wafer. In addition, as mentioned above, This can be measured using normal spectral reflectance measurements. Likewise, additional surfaces can be measured by setting the mirror OPD of each additional surface to zero.

[0069] From I measured The functions extracted from (k,z) related to z (i.e. the position of the mirror) for two positions (such as the top OPD scene and the bottom OPD scene) are as follows:

[0070]

[0071] Each of these functions includes a fully coherent term (as described below each equation) and a partially coherent term.

[0072] The fully coherent terms may then be extracted, step 540, such as by converting to the pulse time domain where the spacing of the surfaces is clear, and then removing data associated with non-OPD surfaces.

[0073] That is, E 1 (k) and E 2 (k) Both can be converted to the pulse time domain by Fourier transform. The impulse response from the non-OPD matching portion can then be removed, and the impulse time domain function can be converted back to the k domain, providing the fully coherent portion of the interferometric field. Additional details of editing the field in the frequency domain / wave number domain by transforming to the impulse response domain are described in PCT Patent No. PCT / IB 2022 / 050774, cited above and incorporated herein by reference.

[0074] As described above, the process may be repeated for additional OPD matching surfaces (represented as step 550).

[0075] Finally, at step 560, the multiple k-domain coherent field spectra can be summed together to provide the fully coherent portion of the measured interferometric spectrum. For two surfaces of interest, the summation will be a function of the two coherence terms listed above:

[0076]

[0077] For additional surfaces, the sum will be a function with additional terms, the first term s1 indicating the reflective surface closest to the light source (i.e. the "top" surface), and subsequent terms (s2, s3, ...) being deeper surfaces, as shown below:

[0078]

[0079] Then, E Coherent (k) can be used for various applications in the wafer production process, such as characterizing structures, thereby providing greater stability and detail than can be provided by decoherence interferometry spectroscopy. For example, in one application of process 500, E can be derived for a reference wafer. Coherent (k), and subsequently, in production, the E of the wafer can be measured Coherent (k) to verify that their structure conforms to the expected reference.

[0080] It should be noted that in order to derive E Coherent (k) The two or more surfaces being measured are usually surfaces whose reflections are partially coherent with each other. When the distance separating the surfaces of interest is so large that the reflections from the surfaces are incoherent (γ' = 0), for a given OPD matching surface, it is not necessary to measured The coherent fields can be partially separated from the z-dependent terms of (k,z). In addition, the coherent reflections from OPD-matched surfaces can be analyzed separately, without combining multiple surfaces into a single field when characterizing the structure.

[0081] It should be understood that the processing elements shown or described herein are preferably implemented by one or more computers in computer hardware and / or computer software contained in non-transitory computer-readable media in accordance with conventional techniques, such as using computer processors, memories, I / O devices, and network interfaces coupled via a computer bus or alternative connection means.

[0082] Unless otherwise specified, the term "processor" or "device" is intended to include any processing device, such as, for example, a processing device including a CPU (central processing unit) and / or other processing circuitry (e.g., a GPU), and may refer to more than one processing device. Various elements associated with a processing device may be shared by other processing devices.

[0083] The term "memory" as used herein is intended to include memory associated with a processor or CPU, such as, for example, RAM, ROM, fixed memory devices (e.g., hard drive), removable memory devices (e.g., floppy disk, tape), flash memory, etc. Such memory may be considered a computer-readable storage medium.

[0084] Additionally, the phrase “input / output device” or “I / O device” may include one or more input devices (e.g., keyboard, mouse, scanner, HUD, etc.) for inputting data to the processing unit, and / or one or more output devices (e.g., speakers, displays, printers, HUD, AR, VR, etc.) for presenting results associated with the processing unit.

[0085] Embodiments of the present invention may include systems, methods, and / or computer program products. A computer program product may include (one or more) computer-readable storage media having computer-readable program instructions thereon for causing a processor to perform aspects of the present invention.

[0086] Computer readable storage medium can be a tangible device that can retain and store instructions for use by instruction execution devices. Computer readable storage medium can be, for example, but not limited to, electronic storage device, magnetic storage device, optical storage device, electromagnetic storage device, semiconductor storage device, or any suitable combination of the foregoing. A non-exhaustive list of more specific examples of computer readable storage medium includes the following: portable computer floppy disk, hard disk, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM or flash memory), static random access memory (SRAM), portable compact disk read-only memory (CD-ROM), digital versatile disk (DVD), blue light, magnetic tape, holographic memory, memory stick, floppy disk, mechanical encoding device (such as punch card or a convex structure in a groove with instructions recorded thereon), and any suitable combination of the foregoing. Computer readable storage medium as used herein should not be interpreted as transient signal itself, such as radio wave or other free propagating electromagnetic wave, electromagnetic wave propagated by waveguide or other transmission medium (for example, light pulse by optical fiber cable), or electrical signal transmitted by wire.

[0087] The computer-readable program instructions described herein can be downloaded from a computer-readable storage medium to a corresponding computing / processing device, or downloaded to an external computer or external storage device via a network (e.g., the Internet, a local area network, a wide area network, and / or a wireless network). A network adapter card or network interface in each computing / processing device can receive computer-readable program instructions from a network and forward the computer-readable program instructions for storage in a computer-readable storage medium in the corresponding computing / processing device.

[0088] The computer-readable program instructions for performing the operation of the present invention can be assembly instructions, instruction set architecture (ISA) instructions, machine instructions, machine-related instructions, microcode, firmware instructions, state setting data, or source code or object code written in any combination of one or more programming languages ​​(including object-oriented programming languages ​​such as Java, Smalltalk, C++, etc., and conventional process programming languages ​​such as "C" programming language or similar programming languages). The computer-readable program instructions can be executed completely on the user's computer, partly on the user's computer as an independent software package, partly on the user's computer and partly on a remote computer or completely on a remote computer or server. In the latter case, the remote computer can be connected to the user's computer through any type of network (including a local area network (LAN) or a wide area network (WAN)), or can be connected to an external computer (for example, by using the Internet of an Internet service provider). In some embodiments, the electronic circuit system including, for example, a programmable logic circuit system, a field programmable gate array (FPGA) or a programmable logic array (PLA) can execute the computer-readable program instructions by utilizing the state information of the computer-readable program instructions to personalize the electronic circuit system, thereby performing various aspects of the present invention.

[0089] Where aspects of the present invention are described herein with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the present invention, it will be understood that each block of the flowchart illustrations and / or block diagrams and combinations of blocks in the flowchart illustrations and / or block diagrams can be implemented by computer-readable program instructions.

[0090] These computer-readable program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device create a device for implementing the functions / actions specified in one or more boxes of the flowchart and / or block diagram. These computer-readable program instructions can also be stored in a computer-readable storage medium, which can guide the computer, programmable data processing device, and / or other device to operate in a specific manner, so that the computer-readable storage medium having the instructions stored therein includes a manufactured product, which includes instructions for implementing various aspects of the functions / actions specified in one or more boxes of the flowchart and / or block diagram.

[0091] Computer-readable program instructions may also be loaded onto a computer, other programmable data processing device, or other device to cause a series of operational steps to be performed on the computer, other programmable device, or other device, thereby producing a computer-implemented process, so that the instructions executed on the computer, other programmable device, or other device implement the functions / actions specified in one or more boxes of the flowchart and / or block diagram.

[0092] Any flow chart and block diagram included herein illustrate the architecture, function and operation of the possible implementation of the system, method and computer program product according to various embodiments of the present invention.In this respect, each square block in the flow chart or block diagram can represent a module, segment or part of an instruction, which can include one or more executable instructions for realizing a specified logical function.In some alternative implementations, the function marked in the square block can appear in the order shown in this article.For example, in fact, the two square blocks shown continuously can be performed substantially simultaneously, or these square blocks can sometimes be performed in reverse order, depending on the functionality involved.It will also be noted that the combination of the square blocks in each square block of the block diagram and / or the flow chart illustration and the block diagram and / or the flow chart illustration can be realized by a system based on special hardware that performs a specified function or action or performs a combination of special hardware and computer instructions.

[0093] The description of various embodiments of the present invention has been presented for the purpose of illustration, but is not intended to be exhaustive or limited to the disclosed embodiments. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terms used herein are selected to best explain the principles of the embodiments, practical applications, or technical improvements over technologies found on the market, or to enable those of ordinary skill in the art to understand the embodiments disclosed herein.

[0094] Example

[0095] Examples of the present invention may include the following configurations.

[0096] Example 1 is a method for optical critical dimension (OCD) metrology for characterizing a structure by an interferometric measurement system, when the structure has at least two reflective surfaces, at least one of the reflective surfaces is separated from at least a second reflective surface by a distance greater than a coherence length. The method comprises the following steps:

[0097] a) setting an interferometer mirror of the system at each of a plurality of positions z, wherein at each position the mirror reflection is an optical path difference (OPD) that matches the reflection of the first reflective surface; and measuring the interferometer spectrum I using the mirror at each of the plurality of positions measured ;

[0098] b) Fitting multiple measured interferometer spectra to I measured to solve the equation for the non-z-dependent parameters, leaving the z-correlation function E with coherent and partially coherent terms for wave number k 1 (k); and

[0099] c) Remove E 1 The partial coherence term of (k) provides the coherent field E 1 (k) Coherent , used to characterize the OCD structure of the first reflective surface.

[0100] A second exemplary method of the present invention includes the features of the first exemplary method, and further includes: repeating steps a) to c) at least for a second reflective surface to generate a coherence function E 2 (k) Coherent The second z-correlation function E 2 (k). Then, we can merge E 1 (k) and E 2 The coherent field of (k) derives the total field E Coherent (k), used to characterize the OCD structure.

[0101] By the third exemplary method, assuming that the first surface is the higher surface of the first surface and the second surface, it can be obtained according to equation E Coherent (k)=[E 1 (k)] Coherent +[E 2 (k)] Coherent ·e ik2Δz The derivation of the combined coherence field is performed, where Δz is the difference between the two OPD matching mirror positions of the two surfaces.

[0102] A fourth example of the present invention is a method including the features of any of the above examples and comprising the steps of: removing E by converting each function into the pulse time domain by Fourier transform 1 (k) and E 2 (k), removing the impulse response from the non-OPD matched portion, and transforming the pulse time domain back to the k domain, thereby providing the fully coherent portion of the measured interferometric field, which corresponds to equation Used to characterize OCD structures.

[0103] A fifth example of the present invention is a method including the features of any of the above examples, and wherein: measured The equation is:

[0104]

[0105] Wherein, each term is defined as described above.

[0106] A sixth example of the present invention is a method including the features of any of the above examples, and wherein fitting the plurality of measured interferometer spectra comprises minimizing the following equation:

[0107]

[0108] It should be understood that another example of the present invention is a metering unit configured to implement any of the above method examples. In addition, another example of the present invention is a non-transitory computer-readable medium storing instructions for implementing any of the above method examples.

Claims

1. A method for optical critical dimension (OCD) metrology for characterizing a structure by an interferometric measurement system, wherein, the structure has at least two reflective surfaces, wherein a first reflective surface is separated from at least one second reflective surface by a distance greater than the coherence length, and the method includes: a) Set the interferometer mirror of the system at each of a plurality of positions z, where, at each position, the mirror reflection is matched in optical path difference (OPD) to the reflection from the first reflective surface, and measure the interferometer spectrum I using the mirror at each of the plurality of positions measured ; b) Fit a plurality of measured interferometer spectra to an equation for I measured to solve for the non-z-dependent parameters of the equation, leaving a z-dependent function E 1 (k) having coherent and partially coherent terms as a function of the wave number k; and c) Remove E 1 The partial coherence term of (k) to provide a fully coherent field E 1 (k) Coherent , an OCD structure for characterizing the first reflective surface.

2. The method according to claim 1, further including: Repeat steps a) to c) at least for the second reflective surface to generate a second z-correlation function E 2 (k) Coherent ; and further comprising: combining the fully coherent terms of E 2 (k) and E 1 (k), and calculating the derived field E 2 (k) for characterizing the OCD structure Coherent .

3. The method according to claim 2, wherein, The second surface is deeper than the first surface, and wherein, E Coherent (k) is derived by according to the equation E Coherent (k) = [E 1 (k)] Coherent + [E 2 (k)] Coherent ·e ik2Δz Combining the fully coherent terms, wherein, Δz is the difference between the two OPD matching mirror positions for the two surfaces.

4. The method according to claim 2, wherein, Remove E 1 (k) and E 2 The partial coherence terms of (k) include: transforming each function to the impulse time domain by Fourier transform, removing the impulse response associated with the part that does not match the OPD, and transforming the impulse time domain back to the k domain to provide the fully coherent part of the interferometric field.

5. The method according to claim 1, wherein, For I measured The equation is: wherein, k is the wave number at each interferometer measurement z i is the mirror position for each measurement in the i-th measurement; is the wavelength-dependent reflection phase difference for the respective reflection pairs tb (top-bottom), tm (top-mirror) and bm (bottom-mirror); H is the geometric difference between the interfaces; n is the refractive index of the structure; r m 、r t 、r b are the reflectivities of the mirror, the top surface, and the bottom surface, respectively, and are all functions of k; γ is the coherence for OPD 2nH; γ' is the coherence for OPD 2(nH - z); and R is the real function of a function with an imaginary part.

6. The method according to claim 1, wherein, fitting a plurality of measured interferometer spectra includes: minimizing the following equation:

7. A metrology unit for optical critical dimension (OCD) metrology for characterizing a structure by interferometry, wherein, the structure has at least two reflective surfaces, wherein a first surface is separated from at least one second surface by a distance greater than the coherence length, and the metrology unit includes an interferometer and a processor, wherein the processor includes an associated non - transitory memory having instructions for an execution process, and the process includes: a) Set an interferometric mirror of the system at each of a plurality of positions z, where, at each position, the reflection of the mirror matches the reflection from the first reflective surface in terms of the optical path difference (OPD), and measure the interferometer spectrum I using the mirror at each of the plurality of positions measured ; b) Fit a plurality of measured interferometer spectra to the equation for I measured to solve for the non-z-dependent parameters of the equation, leaving a z-dependent function E 1 (k) with coherent and partially coherent terms as a function of wavenumber k; and c) Remove E 1 The partial coherent terms of (k) to provide a coherent field E 1 (k) Coherent , an OCD structure for characterizing the first reflective surface.

8. The method according to claim 7, further including: Repeat steps a) to c) at least for the second reflective surface to generate a second z-correlation function E 2 (k) Coherent ; and further comprising: combining the fully coherent terms of E 2 (k) and E 1 (k), and calculating a derived field E 2 (k) for characterizing the OCD structure Coherent .

9. The method according to claim 8, wherein, The second surface is deeper than the first surface, and wherein, E Coherent (k) is derived by according to the equation E Coherent (k)=[E 1 (k)] Coherent +[E 2 (k)] Coherent ·e ik2Δz Combining the fully coherent terms, where Δz is the difference between the two OPD matching mirror positions for the two surfaces.

10. The unit according to claim 8, wherein, Remove E 1 (k) and E 2 The partial coherence terms of (k) include: transforming each function to the impulse time domain by Fourier transform, removing the impulse response associated with the non-OPD matching part, and transforming the impulse time domain back to the k domain to provide the fully coherent part of the interferometric field.

11. The unit according to claim 7, wherein, For I measured The equation is: wherein, k is the wave number for each interferometer measurement z i is the mirror position for each measurement in the i-th measurement; is the wavelength-dependent reflection phase difference for the respective reflection pairs tb (top-bottom), tm (top-mirror), and bm (bottom-mirror); H is the geometric difference between the interfaces; n is the refractive index of the structure; r m 、r t 、r b are the reflectivities of the mirror, the top surface, and the bottom surface, respectively, and are all functions of k; γ is the coherence for OPD 2nH; γ' is the coherence for OPD 2(nH - z); and R is the real function of a complex - valued function.

12. The unit according to claim 7, wherein, fitting a plurality of measured interferometer spectra includes: minimizing the following equation:

13. A non - transitory computer - readable medium storing instructions for optical critical dimension (OCD) metrology for characterizing a structure by interferometry, wherein, the structure has at least two reflective surfaces, wherein a bottom surface is separated from at least one top reflective surface by a distance greater than the coherence length, and the instructions include: a) Set an interferometric mirror of the system at each of a plurality of positions z, wherein, at each position, the reflection of the mirror and the reflection from the at least one top reflection surface are matched in optical path difference (OPD), and measure the interferometer spectrum I at the mirror at each of the plurality of positions measured ; b) Fitting a plurality of measured interferometer spectra to an equation for I measured to solve for the non-z-related parameters of the equation, leaving a z-related function E 1 (k) having coherent and partially coherent terms as a function of the wave number k; and c) Remove E 1 The partial coherent terms of (k) to provide a coherent field E 1 (k) Coherent , for characterizing the OCD structure.

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